1. Field of Invention
The present invention relates to an in-situ gettering method for removing metal impurities from the surface and interior of a upgraded metallurgical grade silicon wafer in a reaction chamber in a continuous manner.
2. Related Prior Art
The current industrial solar cell market is mainly based on crystalline silicon. A standard silicon module of solar cells is made on a high-purity silicon wafer. Since much efforts are necessary to produce high-purity silicon wafer, the cost of the standard silicon module is about 50% on the silicon wafer.
There has been proposed physical metallurgy for making upgraded metallurgical grade silicon (UMG-Si) wafers. The physical metallurgy is relatively environmentally friendly, simple and inexpensive. However, the silicon wafer produced by physical metallurgy involves a relatively high concentration of impurities and a relatively short minority carrier lifetime. Hence, a UMG-Si based solar cell is vulnerable to low efficiency and photo-degradation. Therefore, the physical metallurgy silicon wafer is not suitable for solar cell fabrication.
In an epitaxial wafer-equivalent method, an epitaxial silicon layer is deposited on a UMG-Si wafer. It is supposed to produce relatively efficient silicon solar cells at a relatively low cost. The resultant silicon solar cells are however vulnerable to diffusion of metal impurities therein and the efficiency thereof is inevitably affected. Hence, it is an important issue to remove metal impurities from the interior of a UMG-Si wafer.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.